We propose to test the efficacy of reducing television, videotape and videotape and video game use to prevent obesity among third grade children, in a randomized controlled school-based trial. The United States has experienced dramatic increases in obesity among both children and adults. There is a pressing need for innovative interventions to prevent obesity. There has been widespread speculation that television viewing might be one of the most easily modifiable causes of obesity among children. This hypothesis has broad appeal, but has been difficult to validate. We propose an innovative experimental model. In the current environment, in which television viewing is already so prevalent, the question of greatest clinical, practical and policy importance is: Will reducing television, videotape and video game use prevent childhood obesity? As a foundation for this proposal, we have completed two pilot studies that demonstrate the feasibility and potential promise of the proposed study. We propose a school-based randomized controlled trial involving 12 ethnically-diverse elementary schools and approximately 900 third graders. Six schools will be randomly assigned to receive an intervention to reduce television, videotape and video game use and the other six schools will receive an attention-placebo control intervention. The intervention model is derived from principles of Bandura's social cognitive theory and includes a classroom curriculum and parent newsletters. The primary intervention will be delivered throughout the third grade school year. Survey and physical assessments of all children will occur at baseline, at the end of 3rd grade (post- test) and at the beginning and end of 4th grade (4 month and one year follow-ups, respectively). A sub-sample of children will also complete 4 days of activity monitoring and three 24-hour dietary recalls. Parents will be interviewed by phone at baseline, post-test and one-year follow up. We hypothesize that, compared to controls, third grade children exposed to a school-based intervention to reduce time spent watching television and videotapes and playing video games, will significantly reduce their prevalence of obesity over a single school year. The primary outcome measure will be body mass index. We will also test maintenance of the primary outcome over the follow-up period and effects of the intervention on hypothesized mediating variables; amount of television, videotape and video game use, physical activity, cardiorespiratory fitness, dietary fat and calorie intake and eating while watching television.